All 0's in `g_eta_raw`.

[m0wer]

Trying to debug the problem of getting all NaN's from random_forest_error I found that the g_eta_raw array from https://github.com/scikit-learn-contrib/forest-confidence-interval/blob/bfb97c096287e74728132b8c7923590c2ad518e8/forestci/calibration.py#L101 contains all 0's and then the g_eta_main division throws the warning RuntimeWarning: invalid value encountered in true_divide.

This is probably related to #72, #78, #83, #88 and #92.

¿Any idea on why, despite np.exp(np.dot(XX, eta_hat)) and mask having some values different from 0, their product is a null matrix (all values are 0)? Of course it's because one of them is 0 in every pair of components (e.g. np.exp(np.dot(XX, eta_hat)[i] or mask[i] are 0 for every i in range(len(mask))). But why does this situation occur?